Table 43b

Energy Signature Evaluation for the Everglades Mesocosm in Washington, DC

Actual energy Transformity Energy

Suna 2.05 x 1012 1 2.05 x 1012

Tap water" 2.47 x 108 18,199 4.50 x 1012

Electricityd 3.35 x 1011 1 74,0005.83 x 1016

Gase 1.60 x 1012 48,000 7.68 x 1016

a Average insolation for Washington, DC, is approximately 5.50 X 109 J/m2/year (E. P. Odum,

1971). Total solar energy is (5.50 X 109 J/m2/year)(372.1 m2). b Chemical potential energy of water added to the mesocosm = (volume) (density) (Gibbs free energy). Volume used was 53,400 l/year (Lange, 1998). Density = 1000 g/l. Gibbs free energy = 4.62 J/g (H. T. Odum, 1996). c Labor requirements for the mesocosm were 20 h/week or 43.33 d/year multiplied by the Energy use/person of 9.35 X 1013 seJ/day (H. T. Odum, 1996). d Based on power consumption and operational times of all pumps, heaters, fans, etc., the total electrical use was 3.35 X 1011 J/year in the mesocosm. e Based on power consumption and operational time of gas heaters, the total gas use was 1.60 X 1012 J/year in the mesocosm.

the MERL mesocosms, which showed that turbulence had the highest emergy input, which is perhaps appropriate for a pelagic system.

Seeding of Biota

Seeding a microcosm with biota is usually done after physical scaling considerations. For example, Adey and Loveland (1998) included several procedures for introducing biota in their stepwise instructions for microcosm set up (Table 4.2). This can be an intricate task, for example, in setting up a coral reef system, but in other cases the task is simpler, as for an algal mat system described by H. T. Odum (1967):

At times one can strip the mat from the bay bottom and roll it up like a carpet. When the water is completely blown off of a section by the wind so the mat dries, it is not immediately killed and can be reactivated in a day by putting it back into water. It is a transferable package that Dr. Robert Beyers called an "instant ecosystem."

Microcosm design can even be like a cooking recipe as noted by Darnell (1971) for a protozoan culture in a teaching laboratory workbook:

Preparation of Broth: Add 150 gm of dried grass to 2000 ml of distilled water, and boil for 15 minutes. Cool and strain quickly, once through a double thickness of cheesecloth and once through a thick nest of glass wool in a large funnel. Dilute with distilled water to a volume of 3000 ml, and store in a refrigerator until needed.

Preparation of Cultures: To a one-gallon battery jar add 200 ml of broth (shake well before pouring) and 1800 ml of distilled water. Innoculate with 5 gm of pond mud, and stir vigorously. Partly cover with a glass plate (leaving enough of an opening for gas diffusion), and place in a dark incubator set at 30 degrees C. Bring out for class use on the appropriate day, but keep dark at all times. In such cultures succession proceeds rapidly during the first week and more slowly thereafter, and most major changes will have taken place by the end of three weeks.

In general, there are two basic approaches to seeding a microcosm with biota: (1) use of natural assemblages of organisms obtained from local sources, and (2) the gnotobiotic approach of a synthesized system using standard species (as in Taub's SAM). These are fundamentally different approaches that require different degrees of design by the ecological engineer. In essence, when using natural assemblages as a seed source, the ecological engineer relies completely on self-organization to develop the food web and nutrient cycles within the microcosm. However, when using the gnotobiotic approach, the ecological engineer takes on a significant role as designer of the ecological organization within the microcosm. As an example of this design effort, Taub has noted that much trial and error was required to develop her SAM as a useful tool in ecotoxicology.

In addition to these two fundamental seeding approaches, cross-seeding and reinoculation are often-used techniques in setting up microcosms. Cross-seeding involves mixing innocula between replicate microcosms to reduce variability. This is usually done in early stages of the experiment. Reinoculation is done for specific species which do not develop sustainable populations from the initial seeding. This is usually necessary to maintain desirable species, such as target organisms in ecotoxicology work or species characteristic of the natural analog ecosystem in academic modelling experiments.

H. T. Odum has advocated an approach termed multiple seeding for developing a microcosm. In this approach innocula from several natural assemblages are mixed together to provide a species pool which subsequently becomes self-organized into stable, sustainable ecological circuits. This approach speeds up the self-organization process by providing an excess number of species for internal selection of viable circuits of energy flow and nutrient cycling. All of these approaches and techniques for biotic seeding represent an input of genetic information to the microcosm. In that sense they can be considered as part of the energy signature of the system, though, because the actual energies involved with genetic information are so small, this is seldom done. Perhaps techniques of biotic seeding are best thought of as modelling the processes of colonization or immigration that occur in natural ecosystems.

Island biogeography theory is relevant for explaining some of the features associated with the seeding and development of microcosm biota. As noted at the beginning of this chapter, islands have been important experimental units in ecology,

Number of Species Present

FIGURE 4.17 The species equilibrium concept from the theory of island biogeography. I = immigration rate at the beginning of colonization; S = number of species; P = number of species in the species pool available for colonization. (Adapted from MacArthur, R. H. and E. O. Wilson. 1967. The Theory of Island Biogeography. Princeton University Press, Princeton, NJ.)

Number of Species Present

FIGURE 4.17 The species equilibrium concept from the theory of island biogeography. I = immigration rate at the beginning of colonization; S = number of species; P = number of species in the species pool available for colonization. (Adapted from MacArthur, R. H. and E. O. Wilson. 1967. The Theory of Island Biogeography. Princeton University Press, Princeton, NJ.)

and they have been used as metaphors with microcosms. Natural islands are isolated systems, and when small enough, such as the mangrove islands in Florida Bay studied by Simberloff and Wilson (1969, 1970), they are able to be manipulated in experiments. Islands also have been artificially constructed or intentionally fragmented from existing, especially larger systems for experimental purposes. Thus, there are natural similarities between islands and microcosms, and microcosms have been used to test island biogeography theory, as reviewed by Dickerson and Robinson (1985). As an aside, the unintentional fragmentation of forests and other ecosystems into habitat fragments is a major environmental problem affecting landscapes (Haila, 1999; Harris, 1984; Saunders et al., 1991). Impacts occur because the fragments are isolated and surrounded by a nonforest environment and because of the reduction in area of the habitat fragment. In a sense all microcosms suffer from these same impacts.

The basic tenet of island biogeography theory is that the number of species found on an island is determined by the balance between the immigration rate of species reaching the island from an outside species pool and the extinction rate of species on the island (MacArthur and Wilson, 1967). It has been termed the equilibrium model because the number of species on the island is actually a dynamic steady state (or equilibrium) in which the composition is changing but the number is constant. Thus, the intersection of the immigration rate curve and the extinction rate curve represents the equilibrium number of species to be expected (Figure 4.17). This is a simple, elegant model and, as represented in the energy circuit language in Figure 4.18, immigration is seen as an input or energy source to the system.

A more detailed view of the theory covers islands that are small vs. large and close vs. distant from the species pool, which is usually a continent in some sense. It is controversial but still a useful paradigm for understanding ecological organization (see Chapter 5). This theory is applicable to microcosms in terms of the number of species that can be supported by a closed system. In most cases, seeding of a

Extinction

Extinction

FIGURE 4.18 Energy circuit diagram of the species equilibrium concept from the island biogeography.

FIGURE 4.18 Energy circuit diagram of the species equilibrium concept from the island biogeography.

microcosm is done initially in an experiment over a period of time and then it is stopped. This is analogous to cutting off the immigration rate to an island at a certain point in time. When this situation occurs, a reduction in species results because only extinction takes place. While in a simple mathematical model the number of species goes to zero, in a real microcosm some species maintain themselves in a sustainable organization of energy flow and nutrient cycling. The reduction in species upon removing immigration has been termed relaxation, and is a common phenomenon in microcosm development. It represents a self-organization process whereby those species which fail to find roles within the networks of energy and nutrient flows go extinct. The resulting community after extinctions represents a stable set of species that in a sense are preadapted to the microcosm environment, based on their evolutionary histories. Two examples from the Everglades mesocosm illustrate this phenomenon. First, Lange's (1998) study of fish populations within the mesocosm shows a steady decline in species across the different habitats included in the mesocosm from the initial seeding which started in 1987 through an 8-year period (Figure 4.19). The maximum relaxation of species occurred in the marine tanks of the mesocosm where fish richness diversity declined from 25 to 8. A second example is Swartwood's (in preparation) study of a single species, the mangrove tree snail (Littorina angulifera), within the mesocosm. This species was originally seeded into the mesocosm but it went extinct, perhaps due to insufficient humidity or other habitat factors. The species was reintroduced in 1996 along with an attempt at modification of the microclimate. Populations declined after the reintroduction, perhaps converging on a lower density similar to that found in the natural analog. In both of these cases, preadaptation may explain the survivors, in terms of species with the fish community studied by Lange and in terms of individuals with the snail population studied by Swartwood.

The number of species or species diversity supported by a microcosm can be important to the designer for various reasons. Diversity is an important parameter in ecosystems as mentioned earlier in the book. It has been used as an index of ecosystem complexity and has been linked with stability in the most controversial relationship in ecology. The diversity-stability relationship was formally introduced in the 1950s from empirical observations (Elton, 1958) and theoretical explorations (MacArthur, 1955). Basically, the relationship suggests that more species provide more opportunities for the system to adapt to environmental changes and thus diversity promotes stability. In part because it has a strong commonsense appeal, the diversity-stability relationship has long intrigued ecologists, even though the evidence has not been found to be consistent (Goodman, 1975; Johnson et al., 1996;

Was this article helpful?

0 0
Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook


Post a comment